A Macro Level Analysis Assessing the Effectiveness of Slow Speed Zones in Protecting the Florida Man

A Macro Level Analysis Assessing the Effectiveness of Slow Speed Zones in Protecting the Florida Manatee from Collisions with Watercraft

by Tom McGill, P.E.

January 6, 2009 Abstract:

Manatee mortality data recorded over the past three+ decades indicate that on average approximately 24% is watercraft-related. The State (FWC) and Federal (FWS) Wildlife Agencies in settling lawsuits brought by Save the Manatee Club et al promulgated slow speed zones as the primary method of allegedly adding protection for the manatee. It's worth noting that neither FWS nor FWC challenged the suit in Court. Now, after more than 22% of all Florida's manatee inhabited waters in the 27 regulated counties have been restricted at slow or lower speed for years, the average percentage of watercraft-related deaths should have significantly reduced. However, that average has remained relatively constant which puts in question the effectiveness of such slow speed zones. Interestingly, available research explains why such zones are ineffective and may actually put the manatee at higher risk of collision with vessels.

The Manatee

The Florida manatee (Trichechus manatus latirostris), also known as the West Indian manatee, is a large gentle, whisker-faced resident of Florida's waterways. When mature, the average manatee is ~ 10 feet long and can weigh more than 1,500 pounds. The manatee has graceful movements and is anything but dumb and slow. While not as intelligent as the dolphin, the manatee can be trained. Tracking (telemetry) data has shown that manatees can travel up to 30 miles in one day, although some elect to remain in one area for a long time. Contrary to what is generally believed, the manatee can move at a relatively high rate of speed of 21 feet/second (~14 MPH) for short bursts and can sustain 4-5 MPH for extended periods.

Dr. Edmund Gerstein1 and his wife Laura initiated experiments using two young male manatees, (Stormy and Dundee) at the Lowry Park Zoo in Tampa, Florida in 1991. Their first objective was to define an audiogram, i.e., to determine exactly the hearing capabilities of the two manatees under quiescent conditions. The manatees had to be trained to respond to what they didn't hear as well as what they did hear. The Gersteins were the first to train manatees for psychoacoustic testing, so there was a lot of new ground covered. For example, they didn't know the overall visual acuity of manatees nor did they know which modality (what mechanisms do manatees use to perceive and react) or the combination of modalities manatees rely on the most. ________________________ 1 Gerstein, E., Gerstein, L., Forsythe, S. and J. Blue (1999),"The Underwater Audiogram of the West Indian manatee (Trichechus manatus)," Journal Acoustical Society of America Vol. 105, No. 6, pp. 3575-3583. The Argument

Consider the argument that most vessel-manatee collisions are due to the inability of the manatee to hear or detect and locate the on-coming vessel. In other words, the manatee relying on acoustical cues is surprised and doesn't have sufficient time to avoid a collision. In most Florida waterways, the manatee's ability to see a vessel is substantially limited by the turbidity (murkiness) of the water when the manatee is submerged.

Dr. Edmund Gerstein's research established the hearing capability (audiogram) of the manatee and is the platinum yardstick by which all manatee hearing must be assessed. Gerstein found that the manatee has great difficulty hearing below 400 Hz but can detect up to 46 KHz. The optimum hearing is at ~ 16 to 18 KHz. Also, he found in the acoustic environment in Brevard County2 waters that a vessel at slow speed (4 MPH) could not be detected by a manatee until the primary noise source (propeller) was ~ 12 feet from the animal. Slow moving vessels emit lower frequency and much lower amplitude noise than faster moving vessels. A boat on plane traveling at 24 MPH can be heard by the manatee 560 feet away which provides ~ 16 seconds for the manatee to react and avoid the vessel.

Counter intuitively, a slow moving vessel in murky waters has a much higher probability of impacting manatee because the manatee is unaware of the boat until it is upon it and thus has a very short time for collision avoidance. Likewise, the boat operator is normally unaware of the submerged manatee. Accordingly, slow speed zone effectiveness in such typically murky waters is highly questionable.

The Data

Let's look at the data for the State of Florida and two selected counties. In addition to the State level data, the two counties with the largest manatee population were also included for all causes of manatee mortality. Brevard and Lee County account for about 36% (2,450/6,796) of all the manatee mortality and approximately 32% (521/1,640) of the watercraft-related mortality. Also, both Lee and Brevard Counties have ~ 33% of their waters regulated at slow or lower speed while the overall State regulated counties have ~22%. the watercraft-related mortality data from 1986 through 2008 and shows the years in which there were significant red tide deaths. This permits removal of the red tide data in order to compare data in a normalized manner, i.e., without the bias in those years with significant red tide deaths. also compares the average watercraft-related percentage for 3 time periods in which there were different levels of waterway regulation. All three periods show an average of at least 26 % with red tide removed.

All data has been derived from the FWC/FWRI manatee mortality database. ___________________ 2 Gerstein, E.R., Manatees, Bioacoustics, and Boats, American Scientist, April 2002, Vol. 90. Also, test data collected in Brevard county, var. locations, summer 2003; Gerstein, E.R., et al 1999, The Underwater Audiogram of the West Indian Manatee, Journal Acoustical Society of America Vol. 105, No.6, pp. 3575-3583. Discussion

The average watercraft-related percentage of total manatee mortality over the three selected time periods is essentially constant @ ~ 27% after extracting red tide mortalities. Since the watercraft-related mortality is normalized, i.e., expressed as a percentage of total manatee mortality, it is independent of change in manatee population and carcass recovery rate.

Why would the average watercraft-related mortality remain constant before and after slow speed zones were implemented throughout the state over an extended period of time? Could the slow speed zones actually be effective but something is negating the benefit from them? Could something be exactly offsetting the benefit from such zones? What about the increase in the number of registered vessels?

Those who suggest that slow speed zones may be effective because of the stable average percentage for watercraft-related mortality with an increasing number of registered vessels fail to consider that the number of registered vessels temporarily dropped by ~ 8% after the 2004/2005 hurricanes but still the watercraft-related mortality increased. Also, in Brevard County there has clearly been a decrease in boat traffic as evidenced by the Canaveral Locks Log. If the slow speed zones installed in 2002 in Brevard County and elsewhere were effective, shouldn't we expect the average percentage of watercraft-related manatee mortality since 2002 to have reduced at the county and state levels?

These questions probably are what caused the FWC and FWS to admit that they don't know if slow speed zones are effective or not. But, wait a minute. Didn't these same agencies justify the implementation of such zones based on science or was there no cause-effect understanding in those multiple rule promulgations? The federal and state biologists offered their biological opinions and explained the benefits of such zones. Where are those biologists now, and how do they explain the fact that the zones have not decreased the average watercraft-related percentage? If the zones don't work, and the agencies are unable to provide explanations, then shouldn't the zones be removed?

All the available data say that there is no overall improvement in manatee protection from the slow zones when evaluated at a macro level -- particularly, after having implemented those slow speed zones in more than 22% of all manatee inhabited state waters and more than 33% of the waters in the two counties with the largest manatee populations. The data is irrefutable.

Additional Observations

• The average percentage of watercraft-related mortality in the State for the period 2001 to 2008 was 27.3% which represents a small increase compared to the period 1986 to 2001 value of 26.4%. This increase although only 3.4% is consistent with the observation that slow speed zones may very well increase the probability of vessel-manatee collisions in slow speed zones. A comparable increase in the latter period also occurred in Lee County (28.7% to 29.7%) but not in Brevard County.

• Brevard County had a lower average for watercraft-related mortality, 19.5% for 2001 to 2008, than the average of 22.3% for 1986 to 2001. This would seem to indicate that Brevard County slow speed zones were somewhat effective in lowering the average watercraft-related manatee mortality albeit only a few percent. However, a detailed review of Brevard County's manatee mortality in the recent period of 2003 to 2008 indicates a significant increase in perinatal mortalities which inflated the total mortality and thus caused a corresponding reduction in the average percentage for other mortality causes including watercraft-related. If the change in perinatal increase during that period is taken into account, then the relative average watercraft-related mortality percentage is consistent with the earlier period.

• The increase in the perinatal mortality rate in Brevard County is very problematic. It may be indicative of the manatee population approaching Brevard County's habitat carrying capacity. In 2008, perinatal mortalities in Brevard County were 34% of all perinatal mortalities in the State while Brevard County's total manatee mortality was only 19% of the State! Comparatively, Lee County's perinatal rate in 2008 was 7% with total manatee mortality at 13% of the State total.

• In reviewing the potential impact on the average watercraft-related percentage relative to the increasing number of registered vessels3, it was clear that after normalizing the data (number of watercraft-related mortalities per 10,000 registered vessels), the driving factor for increase in mortality was the increasing manatee population and not the increase in vessel registrations.

• The benefits from additional on-water law enforcement of speed zones in reducing watercraft-related manatee mortality would seem to be moot if the speed zones themselves are demonstrated to be ineffective.

Conclusion The data does not support any effectiveness of existing slow speed zones when assessed at the macro level, and the best available science is consistent with that conclusion in that manatees may be at more risk of collision with slow moving vessels traveling in slow speed zones.

Implementing higher speed corridors in all slow speed zones that have a channel and evaluating the manatee watercraft-related mortality percentage at the county level over a subsequent several year period is a reasonable conclusion/plan.

________________ 3 McGill, Tom, Paper entitled, "Watercraft-related Manatee Mortality and Vessel Registration," dated 7/24/08 and revised 01i/07/09. Contact: Capttom@cfl.rr.com.